The tetrahymena p55 protein and its human and yeast homologues (hGCN5 and yGCN5, respectively), are transcriptional adaptor proteins that are involved in both transcriptional activation and nucleasome destabilization. Both of these activities have been recently shown to be strictly dependent on the ability of p55 to acetylate the histone components of nucleasomes. The region of p55 that confers this activity has been mapped to a conserved enzymatic HAT (for histone acetyltransferase) domain. Our laboratory has been interested in the structural basis underlying this activity. To help provide insights into the structure and function of HAT domains, we have prepared crystals of the HAT domain from p55. The crystals grow as pyrimides with typical dimensions of 0.3 x 0.2 x 0.2 mm, and diffract X-rays to a resolution of at least 2.2 [unreadable] using a rotating anode X-ray source. Characterization of these crystals show that they belong to a primitive monoclinic cell, P31 or P32, with cell dimensions a=b=64.10 [unreadable], and c=97.80 [unreadable]. The available beamtime at CHESS (4/29/98-5/1/98 on beamline F-2) allowed us to collect a complete 1.8 [unreadable] MAD data set from seleno-methionine derivatized crystals of the p55 HAT domain. For the data collection we used 3 wavelengths (12600 eV, 12658 eV and 12662 eV) with the inverse-beam method and obtained 2 x 120,of data at each wavelength. Processing of the data with DPS/MOSFILM shows that it is of high quality (Rsym < 4% to 1.8[unreadable] for each wavelength). We are currently using difference Patterson and difference Fourier techniques to locate and refine the heavy atom positions for MAD phasing. Due to time limitations due to beam downtime we were unable to complete our MAD data set for the PLZF-BTB/POZ crystals.